A computer system typically includes a computer bus that operates as a communications interconnect for components of the bus. A typical type of computer bus is the parallel backplane bus, examples of which include Multibus I, Multibus II, NuBus, VMEbus, STD bus, ISA bus, and EISA bus.
To establish communications between bus components, parallel backplane buses typically include address, data, and control lines that are coupled to each of the components of the computer system. Parallel backplane buses also typically include one or more power and ground lines connected to a central system power supply for supplying power to bus components. The central system power supply typically couples common power supply voltages such as .+-.5 volts and .+-.12 volts to the power lines.
Physically, parallel backplane buses may be implemented on a printed circuit board ("PC board") that includes a number of expansion slots, and bus cards having proper connectors may be coupled to the parallel backplane bus via the expansion slots as components of the bus. Once connected to the bus, bus cards are powered by the power and ground lines of the bus.
The voltages supplied by a bus via power lines are typically selected in view of common operating voltages for semiconductor devices that comprise the PC board components. Unfortunately, as semiconductor processing technology improves, the typical operating voltage for semiconductor devices continues to drop in value, and the voltages typically required by bus components may change after a bus architecture is finalized. For example, the operating supply voltage for TTL devices is 5.0 volts wherein the operating supply voltage for CMOS devices is 3.3 volts, and the use of CMOS devices in computer systems is becoming increasingly prevalent. Further, the operating supply voltage for some CMOS devices continues to drop to values such as 2.9 volts and 2.4 volts. The selection of the voltages supplied by a bus may also be influenced by other considerations such as a limited number of pins for each expansion connector.
To allow semiconductor devices that operate at 3.3 volts or less to operate when only 5.0 volts and 12.0 volts is supplied by the bus, many bus cards include buck-type DC-DC converters or other voltage regulation circuitry to convert 5.0 volts (or 12.0 volts) to the reduced voltage required by the semiconductor devices. A bus card may similarly require a charge pump circuit to boost a lower voltage to a higher voltage so that semiconductor devices requiring voltages higher than those provided by the bus may be powered by the bus. One disadvantage of voltage regulation circuitry such as DC-DC converters and charge pumps is that such circuitry requires valuable space on the bus card. Even the most efficient of voltage regulation circuitry may result in some power losses that are manifested as heat, and heat generated by voltage regulation circuitry may cause component failure under the right conditions. Further, the use of voltage regulation circuitry results in ad hoc solutions that may increase the cost of each bus card. Thus, bus card-based solutions to incompatible operating supply voltages are not desirable.
An alternative solution requires providing an additional edge connector for each bus card and the hardwiring of new power lines to the bus card via the additional edge connector. While such a solution allows a lower voltage bus card to operate without the use of voltage regulation circuitry, if a bus card requiring 2.4 volts is plugged into an expansion slot to which only 3.3 volts has been routed, the devices of the bus card may be damaged if no voltage regulation circuitry is included on the bus card. Therefore, merely hardwiring new power lines to an expansion slot is also an ad hoc solution, and it would be desirable to provide a more flexible solution to the problem of bus components that require an operating voltage other than those supplied by the bus.